This specification refers to several published articles. For convenience, these articles are referenced in full in a numbered list at the end of the description and cited by number in the specification itself. The contents of these articles are incorporated herein by reference and the reader is directed to them for reference.
Fundamental difficulties in creating optical systems or components reside in the simple notions of confining, guiding and redirecting optical energy with relatively low loss, while consuming as little area as possible on a wafer or chip bearing the system or component. The area consumed depends directly on the ability of the chosen optical waveguide to redirect light through bending in the plane of propagation. The smaller the radius of curvature, the smaller the area required to change the direction of propagation. However, in general, as the radius of curvature is decreased beyond a minimum radius set by the waveguide design and operating wavelength, light radiates increasingly out of the waveguide, increasing the loss. Thus, in practice, the minimum radius sets a lower bound on the minimum area needed.
Dielectric waveguides, for example, can provide a nearly lossless means for mode confinement when straight (or bent only slightly) but become significantly radiative when bent aggressively [1-4]. When used in this specification in connection with optical waveguides, “bending aggressively” is intended to embrace bending radii that tend to substantially zero (r0→0). A surface plasmon-polariton (SPP) waveguide can support a mode that is long-range (low propagation loss) but has relatively low confinement, such that it radiates significantly when bent aggressively [5-11]; or that has relatively strong confinement and can be bent aggressively but has a relatively high propagation loss [12-16].
Years ago Yoneyama and Nishida disclosed a non-radiative dielectric (NRD) waveguide [17, 18] for use at microwave frequencies. As shown in FIG. 1, labelled PRIOR ART, their NRD waveguide comprises two metal plates with a core of dielectric material between them and air on either side of the core. When dimensioned properly, the NRD becomes non-radiative in the plane of propagation such that low-loss aggressive bends can be implemented.
The NRD has been worked on extensively over the years to produce many microwave integrated circuit devices [19, 20] and the possibility of using a similar structure at optical wavelengths has been considered. However, recent analysis of the NRD (and similar structures) at optical wavelengths [21,22] revealed that the modes experience relatively large propagation loss due to coupling to SPPs, and that they become highly deformed, and effectively unusable, when the waveguide is bent aggressively. The key benefits of the NRD at microwave frequencies are therefore lost when it is rescaled for optical use [22].